Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
131 During completion of the NCHRP 12-70 Project, it be- came apparent that additional work would be required to de- velop simplified recommendations for the seismic design of retaining walls, slopes and embankments, and buried struc- tures. The required work generally occurs in two categories: (1) fundamental research into seismic performance related to specific issues, and (2) testing of recommended procedures described in this Final Report and as set forth in the specifi- cations and commentaries contained in Volume 2. The fol- lowing discussions summarize some of the topics that will require further research or evaluation. 10.1 Ground Motions and Displacements Applicable ground motion criteria have been established by the AASHTO decision to adopt the 1,000-year ground motion maps and the NEHRP-type site factors as a basis for seismic de- sign. This decision on the part of AASHTO resolves many of the uncertainties that existed during this Project and should provide a sufficient basis for the seismic design of retaining walls, slopes and embankments, and buried structures. The re- vised Newmark displacement charts given in this Final Report also provide an up-to-date method of estimating permanent ground displacements suitable for WUS and CEUS. Height- dependent coherency, or wave scattering, factors also were introduced in this Final Report, and these will be useful for seismic design of walls over 20 to 30 feet in height. The following topics in the areas of ground motions and displacement determination appear to warrant either future consideration or development: ⢠Maps are needed from the USGS that provide PGV for the 1,000 year return period. These maps would eliminate the need to use empirical equations based on the 1-second spec- tral ordinates for making the PGV determination and could contribute to simpler estimates of permanent ground displacements. ⢠Simple but rational methods for estimating site factors at locations should be developed for locations where NEHRP site factors may not be appropriate. These locations in- clude deep soil sites located in CEUS, where the frequency characteristics of ground motions in combination with the depth and shear wave velocity of the soil profile make the NEHRP factors inaccurate in some situations. Likewise, lo- cations where thin soil layers (for example, less than 50 feet) occur over rock also may not be adequately modeled by the NEHRP site factors. ⢠An approach for introducing the effects of liquefaction into ground motion computations is needed. Although one-dimensional, nonlinear effective stress computer pro- grams are available, use of these methods is relatively lim- ited. Either simple ground motion adjustment procedures that account for liquefaction should be developed, or easier- to-use, commercially available, effective stress computer programs are needed. In the absence of these methods, it is difficult to properly account for changes in ground motion above sites where liquefaction is predicted. ⢠Revised equations are needed for estimating the site-adjusted PGA in Equations (5-7) and (5-9) at a predetermined per- manent displacement. The current equations cannot be applied by a designer within a spreadsheet analysis procedure to estimate limiting PGA values if the displacement (d ) is specified. ⢠Additional evaluations should be conducted to confirm that the wave scattering factors described in Chapters 6 and 7 are applicable for a variety of site, retaining wall, and slope conditions. 10.2 Retaining Walls A relatively simple methodology was identified during this work for the seismic evaluation of retaining walls. This method- ology was based on either M-O equations for cases where soil is homogenous behind the retaining structure, or a more C H A P T E R 1 0 Recommendations for Future Work
132 generalized limit equilibrium method using conventional slope stability software, for cases involving layered soils. Charts that included the effects of soil cohesion on seismic active and pas- sive pressures were developed. A key consideration within the methodology was the amount of movement that would develop or could occur during seismic loading, and how this movement would affect the seismic demand on the retaining wall. A number of retaining wall topics were identified as requir- ing further evaluation or investigation. These topics fall into two categories: (1) generic issues and (2) wall-specific issues: 1. Generic issues, relating to the demand and capacity evaluations ⢠Simplified methods of estimating seismic passive earth pressures, particularly for cases involving cohesion, should be developed. Rigorous procedures involving the use of log spiral methods are recommended and charts showing typical results are provided. However, the log spiral approach to passive pressure determination is not easily performed, and in the absence of simple log spiral methods, the designer is likely to resort to less accurate Coulomb or even Rankine methods of estimating passive earth pressures. ⢠The potential for shear banding in cohesionless soils lim- iting the development of seismic active earth pressures needs to be researched. This idea has been suggested by Japanese researchers and by some researchers in North America (for example, R. J. Bathurst and T. M Allen) as potentially limiting the development of seismic earth pressures. The concept is that failure during seismic loading will occur along the same failure surface as de- veloped during static active earth pressure mobilization, rather than changing to some flatter slope angle. This mechanism would limit the development of seismic active earth pressures to much lower values than cur- rently calculated. Unfortunately, the amount of infor- mation supporting this concept is currently limited, though it does appear to have some promise. 2. Wall-specific issues ⢠The inertial force associated with the soil mass above the heel of a semi-gravity cantilever wall remains a de- sign issue. The recommendations in this report assume that the only seismic force that must be considered is the incremental earth pressure from the active failure wedge, and that the soil mass above the heel of the wall does not provide any additional seismic load to the stem of the wall. Detailed finite element analyses could help resolve this issue. ⢠Several issues were identified for MSE walls, including the amount of inertial mass that should be considered for sliding analyses and for the internal design of the re- inforcing system. The approach taken during this Proj- ect was to assume that the entire mass within the rein- forcing strips would respond as a rigid mass, and there- fore should be included within the sliding analyses and internal stability evaluation. This approach can lead to very large inertial forces, which may not develop be- cause of the flexibility of the MSE wall system. As noted in the section on MSE wall design, there are also signif- icant issues regarding the approach used to estimate tensile forces in the reinforcement during internal sta- bility evaluations, and there is a need to upgrade the two standard software packages, MSEW and ReSSA, once a consensus is reached on the approach used to de- sign MSE walls. Part of the design issue associated with MSE walls is how to properly account for the flexibility of the wall system in the method of analysis being used. Additional research on the use of the generalized limit equilibrium approach and evaluation of deformations to define wall performance also is needed. ⢠Nongravity cantilever walls and anchored walls both in- volved a similar question on whether movement of the soil wedge behind the retaining wall will be sufficient to allow use of a lower seismic coefficient. For both wall types the approach being recommended, assumes there is no amplification of ground motions behind the re- taining wall and that the wall will displace enough to sup- port using a seismic coefficient in design that has been reduced by 50 percent. The potential for amplification of forces to values higher than the free-field ground mo- tions is a particular concern for the anchored walls. The process of pretensioning each anchor to a design load ties the soil mass together, and though the strands or bars used for prestressing can stretch, there is a fun- damental question whether the wall-tendon-grouted anchor zone can be simplified by eliminating any inter- action effects. ⢠Whereas soil nail walls appear to be relatively simple in terms of overall seismic design, there are still fundamen- tal questions about the development of internal forces within the soil mass during seismic loading. These ques- tions include whether the internal forces are transferred to the nails in the same manner as during static loading. The AASHTO LRFD Bridge Design Specifications also needs to be supplemented with specific discussions on the static design of soil nail walls, and then these static provisions need to be reviewed relative to provisions appropriate for seismic loading. 10.3 Slopes and Embankments The seismic design of slopes and embankments was identi- fied as a more mature area of seismic design, where both sim- plified limit equilibrium and displacement-based approaches
are conventionally used to investigate the seismic stability of engineered slopes and natural cut slopes. The primary topics that require further study area are as follows: ⢠The appropriate liquefaction strength to use when assessing the stability of slopes comprised of or resting on liquefiable materials needs to be established. A number of issues about the liquefaction strength remain difficult to quantify, and these difficulties lead to uncertainty in design. Issues in- clude simple methods of estimating the liquefied strength at locations involving sloping ground (that is, where a static shearing stress is imposed) and appropriate liquefied strength values for cohesionless soil where limited defor- mations occur. Included within this topic is the potential for ratcheting movements and how to adequately represent this mechanism. ⢠Stability of rock slopes requires further evaluation. This topic was not addressed during this Project because of the com- plexity of the problem. Although a transparent approach does not seem possible, some additional guidance on factors to consider when conducting a site-specific seismic evalua- tion would assist designers when they have to deal with rock slope stability. 10.4 Buried Structures The buried structures portion of the Project provided design equations for rigid and flexible culverts and pipelines subjected to TGD. Guidance also was provided on design considerations for PGD such as might occur during liquefaction-induced lateral spreading or seismic-induced embankment failures. Sec- tion 12 of the current AASHTO LRFD Bridge Design Specifica- tions does not cover seismic response of culverts and pipelines, and therefore the developments summarized in this report address a current deficiency in the AASHTO Specifications. The treatment of buried structures in this Project was relatively limited in terms of levels of effort, and additional studies will be required to advance design methods for buried structures: ⢠Methods suggested in Chapter 9 need to be tested on a range of pipe configurations, ground conditions, and earthquake shaking levels to confirm that the recommended approaches for TGD design are practical. Experimental studies involv- ing TGD also are needed to confirm the validity of the nu- merical methods being suggested. ⢠Further guidance needs to be developed for modeling pipeline behavior in conditions where PGD occurs. These developments include appropriate spring constants to use in modeling soil-pipe interaction for moving ground conditions. ⢠The seismic effects of transient racking/ovaling deforma- tions on culverts and pipe structures need to be incorpo- rated into the updated CANDE analysis. It is anticipated that an option will be required in the CANDE program to allow ground displacement profile as a loading input to the CANDE analysis. 10.5 Need for Confirming Methods One clear conclusion from this Project was that various methods are available to the designer to use for the seismic design of retaining walls, slopes and embankments, and buried structures. These methods range from simple equations to advanced numerical methods. The focus of this Project has been to develop simple methods of analysis suitable for use in AASHTO LRFD Bridge Design Specifications. By focusing on simple methods, a number of simplifying assumptions and approaches had to be taken. Whereas checks and then exam- ple problems were completed to test these proposed methods, additional test cases will be required to identify areas where the simplifications are not appropriate, are too conservative, or lack conservatism. For example, test cases involving advanced numerical methods or experimental centrifuge testing could be used to confirm the simplified methods. 133
